System and method for detecting defective back-drills in printed circuit boards

ABSTRACT

The present invention provides a method for detecting failed back-drills in PCBs in the process of fabricating a PCB so that the failed back-drill can be screened out or repaired. The present invention accomplishes this by adding a short to ground connection for every back-drill via that will be cut when the back-drill removes the via stub. If the back-drill is bad or failed the short to ground will fail the subsequent electrical tests. The PCB can be repaired by re-drilling the hole or via. The present invention allows for detecting failed back-drills with easy detection in the manufacturing stage using standard equipment and test procedures. This process creates a simple pass-fail measurement that uses an existing common test process to catch failed back drills in the PCB fabrication facility. This allows for easy and cost-effective repair and guarantees back-drill failures do not pass into the field.

BACKGROUND 1. Field

The present invention relates to back drilling in printed circuit boards (PCBs). In particular the present invention relates to detecting poor or failed back-drills in printed circuit boards.

A failed back-drill in a PCB is one that does not remove all of the metal in the drilled hole or via. This can be due to any of a variety of reasons during the PCB fabrication process such as a drill-misalignment. FIG. 1 Illustrates drill misalignment. A failed drill-back will leave a conductive path of some or all of the length of the drilled via. The via stub created by the failed drill-back results in a degradation of high frequency signals in that path in the PCB.

2. The Related Prior Art

Common PCB testing is accomplished by measuring opens and shorts by employing a flying-probe test. However, these testers are not able to detect a failed back-drill. It is necessary to utilize high-speed test tools such as a Vector Network Analyzer or a Time domain Reflectometer in order to see the failed back-drill but these tools are expensive, difficult to implement in scale and produce difficult to analyze results. Visual inspection has a limited 1820 Application for Method for Detecting Failed or Poor Back Drills in Printed Circuit Boards success rate of detecting back-drill problems and 3 D X-rays machines are both expensive and unreliable.

SUMMARY

It would therefore be advantageous to have a novel method for detecting failed or poor back-drills in PCBs that avoids the drawbacks of the aforementioned prior art proposals.

The present invention provides a method for detecting failed back-drills in PCBs early in the process of fabricating a PCB so that the failed back-drill can be screened out or repaired.

The present invention accomplishes this by adding a short to ground connection near the signal layer for every back-drill. This short to ground connection is cut by a successful back-drill. If the back-drill is bad or failed it will leave the connection to ground and fail the electrical test with a short to ground being detected. The PCB can be repaired by re-drilling the hole or via.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A shows the placement of the short to ground near a signal layer of the PC;

FIG. 1 B shows a good back-drill in a PCB;

FIG. 1 C shows a bad back-drill in a PCB;

FIG. 2 A shows the selected, configured or design PCB with a transmission line consisting of a via, a signal trace, another a via, and two stubs before back-drilling

FIG. 2 B shows shorting the signal trace to ground in a hole or a via of the PCB;

FIG. 2 C shows a bad back-drill failure due to drill shift;

FIG. 2 D shows a bad back-drill failure due to drill wander;

FIG. 2 E shows a bad back-drill failure due to shallow drill;

FIG. 2 F shows one example of electrical connection to ground failure;

FIG. 2 G shows another example of electrical connection to ground failure;

FIG. 2 H shows another example of electrical connection to ground failure;

FIG. 2 I illustrates a back-drill cutting an electrical connection to a power plane;

FIG. 2 J shows two back-drills that removed a two via stubs;

FIG. 3 illustrates the methodology of the present invention; FIG. 3 shows one method of how the invention can be automated in the PCB design process and integrated with manufacturing the PCB for opens and shorts to determine if, among other things, all of the back-drills removed all of the short to ground connections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The referenced elements for the present invention include:

1. PCB 5.

2. Via or hole 8.

3. Shorting trace 10

4. Signal level or trace or stripline 16

5. Back-drill 15.

6. Ground/power plane 23.

7. Transmission line or signal path 11.

Referring now to the drawings, FIG. 1 A, in accordance with the teachings of the present invention illustrates the methodology of the present invention wherein in the PCB design a short to ground connection 10 (hereinafter referred to as a shorting trace 10) is added into the design a programmed distance from the signal trace 16 that electrically connects or shorts the signal via 8 to ground. Thus there is a placement of the shorting trace 10 near a signal trace 16 of the PCB 5. Therefore if, for example, a 10 mil±5 mils back-drill 15 is drilled in a hole or via 8 of the PCB 5, the shorting trace 10 is placed on the first ground layer 16 mils past the signal trace or strip line 16. When a successful back-drill operation is performed the back-drill 15 breaks the electrical connection between signal trace 16 and ground. If the back-drill 15 fails it will leave a path of conductive material in the signal via 8 that will still short signal trace 16 to ground 10 at that sign at that signal trace 16. A failed back-drill is illustrated in FIGS. 2 H, 2 G and 2 I. A good back-drill is shown FIG. 1 B FIG. 2 J.

FIG. 2 A shows a PCB with transmission line path 11 from top left to bottom right. The transmission line is a via 8, a trace or stripline 16, and a via 8. Via stubs 10 are also present

FIG. 2 B shows a shorting trace 6 connecting the via 11

FIGS. 2 C-2 E show common back-drill failure modes.

FIG. 2 C shows a back-drill failure due to drill shift. FIG. 2 E shows a bad back-drill failure due to drill wander. Drill wander is when a drill does not drill straight down into the PCB 5 but instead curves away from a straight up or down path FIG. 2 F shows a back-drill failure due to shallow drill. A shallow drill is one that leaves a stub 10 longer than the maximum stub length allowed.

FIGS. 2 F-2 H show common back-drill failure modes with a shorting trace 6 connecting signal to ground.

FIG. 2 I shows a back-drill 15 that cuts the shorting trace signal to ground or power plane 23 connection.

FIG. 2 J shows a PCB 5 with back-drill 15 that removed the via stubs 8.

FIG. 3 illustrates the methodology of the present invention. In FIG. 3 illustrates a first step of the methodology of the present invention in which a design or configured PCB 5 is selected. Next FIG. 3 shows a second step 18 in which shorting traces 10 are added to back-drilled vias 8 in the PCB 5 design.

FIG. 3 illustrates a third step of the methodology in which the PCB 5 is designed 17 and subsequently the electronic design files are modified using automated software 18 to add shorting traces for each back-drilled via. The PCB 5 is then manufactured 19 using common PCB manufacturing techniques. It is then tested 20 to verify each transmission line 11 is electrically isolated from ground 23 to confirm the back-drill 15 cut the shorting trace confirming a good back-drill. If there is a short then the back-drill 15 has failed and the PCB 5 must be discarded or repaired.

The methodology of the claimed invention is preferably software implemented in the following steps:

-   -   1. Execute the program and select the design files     -   2. There are three sections         -   Back Drill         -   Compare IPC         -   Compare Shorts     -   3. The first section “Back Drill” will update all hack drill         vias to create intentional short to ground     -   4. The second section “Compare IPC” will compare the board files         before and after intentional short to verify correct         functionality     -   5. The third section “Compare Shorts” compares the error         checking output files to the expected errors that are generated         from this software. The shorting of a signal via to ground will         create a DRC error that other computer aided manufacturing         software will detect. CAM 350 is an example of Gerber Computer         Aided Manufacturing tool that will detect said errors. This         section will make sure that other unrelated errors are separated         from errors caused by this process     -   B. Back-Drill program details     -   1. User implements the following parameters:         -   a. Via stub length: This value is the minimum stub length             before a short can be added (E.g. 10 mils)         -   b. Ground net: This is the name of the ground net in the PCB             design software. (E.g. “VSS”)         -   c. Copper Web: This value is the copper web in the custom             antipad allowing ground copper to flow in the back drill             vias by default the tool will use a value of 5 mils.         -   d. Copper Diameter: This is the copper diameter around the             hole.         -   Program steps include:     -   2. Execute program:         -   a. Load PCB design file         -   b. Loop through all back drilled via locations         -   c. Start at trace depth, define this as position 0         -   d. Subtract “via stub length” from this position 0 (E.G.             0−10=−10)         -   e. Start at this calculated position and look for closest             “Ground net” copper plane while moving away from the trace             location. (E.G. “VSS layer located at −16.5 mils”)         -   f. Add shorting feature defined by “Ground Web” and “Copper             Diameter” (E.G. see picture)         -   g. Rename design file via name to customer name indicating             anti-pad modification. <image.png>     -   h. Create report file indicating changes made

The methodology of the present invention is assisted from tools improvements in a typical PCB design flow. All common PCB design tools have a design rule check, also known as DRC that looks for nets that are shorted together as well as other error checking. A program or script adds the shorting trace 10 at all back-drill 15 locations and adjusts the depth of the shorting trace 10 by the tolerance of the back-drill 15 and the depth of the signal trace 6.

Once the shorting traces 10 are added the common PCB design or Gerber files then computer added manufacturing, A.K.A. CAM, tools will show DRC errors. The back-drill shorting trace 10 DRC flags must be identified and screened out so DRC flags for legitimate errors are caught and fixed. It is understood that although copper web is preferably used any suitable electrically conductive metallic material can be used.

While presently preferred embodiments have been described for purposes of the disclosure, numerous changes in the arrangement of method steps and apparatus parts can be made by those skilled in the art. Such changes are encompassed within the spirit of the invention as defined by the appended claims 

1. A method, for detecting failed back-drills in a printed circuit board (PCB), the method comprising: accessing PCB design; selecting a non-ground via of the PCB design; adding a shorting trace from the non-ground via to ground in the PCB design; accessing a PCB manufactured according to the PCB design; and electrically testing the PCB to determine if the non-ground via is shorted to ground.
 2. The method according to claim 1 wherein the shorting trace comprises a stub length dimension of 10 mils plus or minus 5 mils.
 3. The method according to claim 1 wherein the shorting trace comprises a stub length dimension of 10 mils plus or minus 5 mils.
 4. The method according to claim 1 wherein the shorting trace is located on a power layer of the PCB.
 5. The method according to claim 1 wherein the shorting trace is located on a signal layer of the PCB.
 6. The method according to claim 1 wherein the PCB comprises a plurality of shorting traces on a plurality of layers of the PCB.
 7. The method according to claim 1 wherein the non-ground via of the PCB has been back-drilled.
 8. A method of designing a printed circuit board (PCB), the method comprising: selecting a PCB design in a PCB design system; and adding a shorting trace from a back drill signal via to a ground node in the PCB design to create an updated PCB design, and wherein back drilling of the signal via during manufacture of a PCB according to the updated PCB design is operable to open the shorting trace.
 9. The method of claim 8 further comprising: comparing error checking output files produced by the PCB design system of the PCB design to expected errors generated from said PCB design system, wherein a shorting of the signal via to ground creates an error profile that will be detected by detection software in said PCB design system to identify unrelated errors apart from the expected errors form the shorting trace.
 10. The method of claim 8 wherein said PCB design system comprises a design rule check (DRC) module that inspects nets that are shorted together.
 11. The method of claim 10 wherein the PCB design system is configured to adjust a PCB layer comprising the shorting trace by a tolerance of the back-drill and a thickness of the signal trace.
 12. The method of claim 10 wherein the DRC module is configured to suppress display of errors resulting from the shorting trace.
 13. The method of claim 8 wherein the shorting trace comprises a stub length of 10 mils plus or minus 5 mils.
 14. The method of claim 8 wherein the shorting trace is placed on a power or ground plane layer of the PCB design.
 15. A printed circuit board (PCB) comprising: a plurality of layers comprising conductive patterns; a plurality of vias configured to electrically couple traces formed on different layers of said plurality of layers; and a shorting trace from a non-ground via to a ground node.
 16. The printed circuit board (PCB) of claim 15 wherein the non-ground via is physically and electrically isolated from the round node due to back-drilling of the PCB.
 17. The printed circuit board (PCB) of claim 15 wherein subsequent to back-drilling of the PCB, the non-ground via is electrically coupled to ground.
 18. The printed circuit board (PCB) of claim 15 wherein the shorting trace comprises a stub length of 10 mils plus or minus 5 mils.
 19. The printed circuit board (PCB) of claim 15 wherein the shorting trace is disposed on a power or ground plane layer of the PCB.
 20. The printed circuit board (PCB) of claim 15 wherein the shorting trace is disposed on a signal layer of the PCB. 